There has heretofore been known a radio terminal connectable to multiple networks employing different radio communication schemes (including “1×EV-DO” in compliance with CDMA2000 and “WiMAX” in compliance with IEEE802.16e, for example).
The radio terminal includes a mobility manager (MIH user; Media Independent Handover User) configured to manage mobility between the multiple networks, and multiple link controllers each configured to control a radio link established between its own terminal and the corresponding network. Moreover, there is a proposal to provide a handover controller (MIHF; Media Independent Handover Function) which is provided between the mobility manager and the multiple link controllers, and which is configured to control handovers between the multiple networks (Non-patent Document 1, for example).
To be more precise, the mobility manager notifies the handover controller of types (SINR, RSSI, DRC, Tx_Power, and DRC_Lock, for example) of multiple link parameters and information (MIH_Configure.request) indicating multiple thresholds set for each of the link parameters. Here, the thresholds include a first threshold (Initiate Action Threshold) for judging whether or not to perform preparation for a handover (Initiation Action), a second threshold (Execute Action Threshold) for judging whether or not to execute the handover is (Execute Action), and a third threshold (Rollback Action Threshold) for judging whether or not to cancel the preparation for the handover (Rollback Action).
The handover controller notifies the link controller controlling a radio link already established, of information (Link_Configure_Threshold.request) indicating the types of the multiple link parameters and the multiple thresholds set for each of the link parameters.
When a link parameter value becomes worse than the first threshold, the link controller notifies the handover controller of information (Link_Parameters_Report.indication) indicating the link parameter value. The handover controller notifies the mobility manager of information (MIH_Link_Parameters_Report.indication) indicating the link parameter value.
Based on this notice, the mobility manager instructs preparation of a handover, i.e., an establishment of a radio link which is yet to be established.
Subsequently, when the link parameter value becomes worse than the second threshold (which is a value representing a situation where a radio environment is worse than that of the first threshold), the link controller notifies the handover controller of the information (Link_Parameters_Report.indication) indicating the link parameter value. The handover controller notifies the mobility is manager of the information (MIH_Link_Parameters_Report.indication) indicating the link parameter values.
Based on this notice, the mobility manager instructs execution of the handover, i.e., switching of the networks to which the radio terminal is connected.
In the meantime, when the link parameter value becomes better than the third threshold, the link controller notifies the handover controller of the information (Link_Parameters_Report.indication) indicating the link parameter value. The handover controller instructs cancellation of the preparation for the handover (Initiation Action).
As described above, since the link controller only has to perform threshold judgment, a process load on the link controller is reduced.    Non-patent Document 1: IEEE802.21 Draft Standard (P802.21/DO4.00)